The invention relates generally to an organic Rankine cycle energy recovery system, and more particularly to a direct evaporator apparatus and method for energy recovery employing the same.
So called “waste heat” generated by a large number of human activities represents a valuable and often underutilized resource. Sources of waste heat include hot combustion exhaust gases of various types including flue gas. Industrial turbomachinery such as turbines frequently create large amounts of recoverable waste heat in the form of hot gaseous exhaust streams.
Organic Rankine cycle energy recovery systems have been deployed as retrofits for small- and medium-scale gas turbines, to capture waste heat from the turbine's hot gas stream and convert the heat recovered into desirable power output. In an organic Rankine cycle, heat is transmitted to an organic fluid, typically called the working fluid, in a closed loop. The working fluid is heated by thermal contact with the waste heat and is vaporized and then expanded through a work extraction device such as a turbine during which expansion kinetic energy is transferred from the expanding gaseous working fluid to the moving components of the turbine. Mechanical energy is generated thereby which can be converted into electrical energy, for example. The gaseous working fluid having transferred a portion of its energy content to the turbine is then condensed into a liquid state and returned to the heating stages of the closed loop for reuse. A working fluid used in such organic Rankine cycles is typically a hydrocarbon which is a liquid under ambient conditions. As such, the working fluid is subject to degradation at high temperature. For example, at 500° C., a temperature typical of a hot heat source gas from a turbine exhaust stream, even highly stable hydrocarbons begin to degrade. Worse yet, a hydrocarbon working fluid useful in an organic Rankine cycle energy recovery system may begin degrade at temperatures far lower than 500° C. Thus, the use of an organic Rankine cycle energy recovery system to recover waste heat from a gas turbine system is faced with the dilemma that the temperature of the exhaust is too high to bring into direct thermal contact with the working fluid of the organic Rankine cycle energy recovery system.
In order to avoid the aforementioned issue, an intermediate thermal fluid system is generally used to convey heat from the exhaust to an organic Rankine cycle boiler. In an example, intermediate thermal fluid system is an oil-filled coil which moderates the temperature of the working fluid in the organic Rankine cycle boiler. However, the intermediate thermal fluid system can represent significant portion of the total cost of an organic Rankine cycle energy recovery system. Furthermore, the intermediate thermal fluid system both increases the complexity of the organic Rankine cycle energy recovery system and represents an additional component the presence of which lowers the overall efficiency of thermal energy recovery.
Therefore, an improved organic Rankine cycle system is desirable to address one or more of the aforementioned issues.